Ultrapotassic rocks of the Christopher Island Formation (Baker Lake basin) were emplaced across an enormous area (240 000 km2 minimum) of the western Churchill Province ca. 1.83 Ga. These rocks extend across the Snowbird zone, a geophysical feature postulated by others to represent a Paleoproterozoic suture that welded the Rae and Hearne domains. Minette dikes and flows of the Rae and Hearne domains display identical ϵNd, 1830 values and incompatible element patterns, and thus appear to have originated from a common lithospheric-mantle source. Christopher Island Nd model ages cluster at 2.8 Ga, and ϵNd data from one Archean lamprophyre and three 2.45 to ca. 2.2 Ga mafic suites suggest that enriched lithospheric-mantle sources beneath both the Rae and Hearne domains existed well before ca. 1.83 Ga, inconsistent with Paleoproterozoic suturing along the Snowbird zone. In contrast to commonly invoked models that envisage melting of local enriched domains, Christopher Island ultrapotassic rocks appear to have originated from an extensive reservoir. We suggest that such a reservoir was created during an Archean metasomatic event, perhaps owing to flat subduction, and that it remained in nearly complete isolation until tapped during Paleoproterozoic extension related to squeezing of western Churchill crust between flanking Wopmay and Trans-Hudson orogens.

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